fix(ribbon): center section titles within their content width

Section labels (Draw, Modify, Annotation, …) were left-aligned with a small padding. Set align_x(Center) on the parent column so the label container — still sized to its text width — is centred within the column, which itself remains sized by the tool buttons above. Sections keep their natural content-based width, no wasted space.

src/ui/ribbon/mod.rs

@@ -452,6 +452,7 @@ impl Ribbon {
                             tools_el,
                             container(text(group.title).size(9).color(GROUP_LABEL)).padding([1, 4]),
                         ]
+                        .align_x(iced::Center)
                         .spacing(0)
                         .padding([3u16, 4])
                         .height(Length::Fill)